Definition:
A shot needs to move more slowly than it was captured. Optical flow is the computational process that makes that possible. It analyses the movement of pixels between existing frames and generates new in-between frames, allowing footage to be slowed down, stabilised, or converted to a different frame rate in post-production without visible stuttering.
What It Means in Movies:
When a DP shoots at 24fps and the editor needs to slow that footage to 10 percent speed, there are not enough frames to fill the gap without optical flow generating new ones. The result is smoother than a simple frame repeat but rarely as clean as footage originally captured at a high frame rate. The best slow-motion results still come from shooting at the required frame rate on set.
Why It Matters:
Optical flow is built into every major professional editing platform and is used constantly in post-production work. In areas of complex motion or low image detail, optical flow can produce visible artefacts: ghosting, smearing, and objects that bend when they should not. Knowing when to use it and when to go back to set is part of the post-production craft.
Example:
An editor in DaVinci Resolve uses optical flow to slow a 24fps clip to 10 percent speed for a stylised flashback sequence. The result is usable. Whether it is as clean as footage shot at 240fps on set is a different question, and usually the answer is no.